Production method of high density iron based forged part

ABSTRACT

A high density iron based forged part such as a mechanical part is produced by a method comprising the following steps in the sequence set forth: (a) preparing iron based powder mixture containing iron based metal powder and graphite powder; (b) preliminarily compacting the iron based powder mixture to form a preliminary compact; (c) sintering the preliminary compact in a non-oxidizing atmosphere whose nitrogen partial pressure is 30 kPa or less, at a temperature of 950° C. or more and of 1300° C. or less to form a forming material; and (d) forging the forming material by closed die forging or enclosed die forging to produce a high density forged part.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to improvements in a method of producing or manufacturing an iron based forged part that is suitable for mechanical parts, and more particularly to the method by which reduction of a forging load is achieved improving the density and dimensional precision of the forged part.

[0002] Powder metallurgy technology allows producing or manufacturing a complicatedly shaped part in a near-net shape and with high dimensional precision, resulting in a large reduction in cutting cost. In recent years, to iron based powder metallurgical products (iron based powder products or iron based sintered products), further higher mechanical strength is in demand in order to realize smaller size and lighter weight.

[0003] The iron-based sintered parts (hereinafter referred to as iron based sintered bodies or simply sintered bodies) are generally producing or manufactured according to the following processes. That is, iron-based metal powder is mixed with alloying powder such as graphite powder, copper powder and a lubricant such as zinc stearate, lithium stearate, thereby an iron based powder mixture is prepared. At the next step, the iron based powder mixture is filled in a metallic die followed by compacting, thereby preparing a compacted body. Then, the compacted body is sintered to produce a sintered body. The thus obtained sintered body is, as needs arise, subjected to sizing or cutting to form a product. Furthermore, in the case of high mechanical strength being needed, in some cases, the sintered body is further subjected to carburizing heat treatment or bright heat treatment. The density of the formed body obtained according to such a process is at most in the range of substantially 6.6 to 7.1 Mg/m³. Accordingly, the density of a sintered body obtained from the formed body is to this extent.

[0004] In order to make the iron based powder product (iron based sintered part) stronger in the mechanical strength, it is effective to make the formed body higher in the density and therefrom to obtain a denser sintered part (sintered body). The denser the sintered part (sintered body) is, the less pores are in the part, resulting in an improvement in the mechanical properties such as tensile strength, impact resistance value, and fatigue strength.

[0005] As a forming method for making the density of the iron based powder product (iron-based sintered part) higher, a sintering-cold forging method in which, for instance, a powder metallurgical method and a cold forging method are combined has been proposed in JP-A-1-123005,in which a product having the density substantially close to a true one can be obtained. The sintering-cold forging method is a forming and processing method in which a pre-form (preliminarily formed body) that is obtained by sintering metal powder is subjected to the cold forging followed by re-sintering, and thereby obtaining a final product having a higher density structure. Technology disclosed in JP-A-1-123005 is a sintering cold forging method in which a sintered pre-form (for cold forging) with a liquid lubricant coated on a surface thereof is tentatively compacted in a die followed by applying a negative pressure on the preform to suck and remove the liquid lubricant further followed by a main compacting in the die and still further followed by re-sintering. According to the method, the liquid lubricant that is coated and infiltrates the inside of the pre-form before the tentative compacting is sucked before the main compacting, and accordingly fine pores inside thereof are crushed flatly and eliminated at the main compacting, resulting in a denser final product. However, since the density of an end sintered-product obtained according to the method is at most about 7.5 Mg/m³, there is a limit in the mechanical strength thereof.

[0006] On the other hand, in order to further improve the mechanical strength of the sintered product (sintered body), it is effective to increase a concentration of carbon (C) in the product. In powder metallurgy, it is general to mix graphite powder, as a carbon (C) source, with raw material metal powder. In this regard, the following method may be assumed: Metal powder that is mixed with graphite powder is preliminarily compacted followed by tentative sintering (preliminary sintering) to prepare a forming material (to be formed). Furthermore, the forming material is once more compacted followed by re-sintering, so that a sintered body having higher mechanical strength is obtained. However, when the tentative sintering (preliminary sintering) is applied according to the conventional method, carbon (C) diffuses in an entirety of the forming material at the tentative sintering (preliminary sintering), resulting in an increase of hardness of the forming material. As a result, there is a problem in that when the compacting is once more applied, the forming load becomes very large and deforming properties are deteriorated, resulting in incapability of processing into a desired shape. Accordingly, a product of higher strength and higher density cannot be obtained.

[0007] In order to overcome such problems, for instance, in U.S. Pat. No. 4,393,563, a producing method in which a bearing part is manufactured without applying the forming at a high temperature is disclosed. According to the method, the producing processes includes mixing iron powder and iron alloy powder with graphite powder and a lubricant; forming a powder mixture into a preliminarily formed product followed by tentatively sintering; subsequently applying cold forging that gives at least 50% plastic working followed by sintering, annealing, and rolling; and thereby obtaining a final product (sintered member). According to the technology disclosed in U.S. Pat. No. 4,393,563, when the tentative sintering is performed under the conditions that can suppress graphite from diffusing, higher deformability can be exhibited in the latter cold forging, resulting in lowering the forming load. Although U.S. Pat. No. 4,393,563 recommends the tentative sintering conditions of a temperature of 1100° C. and a time of 15 to 20 min., experiments conducted by the inventors show that under the above conditions, graphite completely diffuses into the preliminarily formed body, and therefore the hardness of raw material for use in the sintering member (preliminarily formed product) becomes very high, resulting in causing difficulty in the latter cold forging.

[0008] In order to overcome such problems, for instance JP-A-11-117002 discloses a metallic powder forming material. The metallic powder forming material is obtained by tentatively sintering a preliminarily formed body that is obtained by compacting metallic powder in which metal powder containing iron as a main component and mixed with 0.3% or more by weight of graphite and that has a density of 7.3 Mg/M³ or more, preferably at a temperature in the range of 700 to 1000° C. The metallic powder forming material has a structure in which in graphite remains in grain boundaries of metal powder. It is disclosed that according to the technology, only an amount of carbon necessary for improving the mechanical strength is dissolved and free graphite is allowed to remain, so that iron powder is inhibited from becoming excessively hard, resulting in obtaining a forming material having low forming load and high deformability at the re-compression forming. However, there remain problems in that although the metallic powder forming material obtained according to the method is highly deformable in the re-compression forming, in a subsequent main sintering process, residual free graphite disappears and, in some cases, leaves long and slender pores.

[0009] Furthermore, in JP-A-2000-303106, a method of producing a sintered body is disclosed. The method includes a step of tentatively sintering, at a certain temperature, a preliminarily formed body that is obtained by compacting metallic powder in which metal powder containing iron as a main component and 0.3% or more by weight of graphite are mixed and has a density of 7.3 Mg/m³ or more, thereby obtaining a metallic powder forming material having a structure in which the graphite remains in grain boundaries of metal powder; a step of re-compressing in which the metallic powder forming material obtained according to the tentative sintering is re-compacted; and a step of re-sintering in which a re-compacted body obtained according to the re-compressing is re-sintered.

[0010] Furthermore, in JP-A-2000-355726, an alloy steel powder re-sintered processed body is disclosed. The alloy steel powder re-sintered body is manufactured by tentatively sintering, at a certain temperature, a preliminarily formed body that is obtained by compacting metallic powder in which alloy steel powder and 0.1% or more by weight of graphite are mixed and has a density of 7.3 Mg/m³ or more, thereby forming a metallic powder forming material having a structure in which the graphite remains in grain boundaries of metal powder; re-compression forming the metallic powder forming material thereby forming an alloy steel powder plasticity-processed body having a densified structure that contains substantially no voids; and re-sintering the alloy steel powder plasticity-processed body at a certain temperature, thereby obtaining the alloy steel powder re-sintered processed body that has a structure to which graphite diffuses out and a structure where graphite remains at a certain ratio in accordance with a re-sintering temperature.

SUMMARY OF THE INVENTION

[0011] According to the technology disclosed in JP-A-2000-303106 and JP-A-2000-355726, a higher density and higher strength sintered body can be obtained. However, according to the technology disclosed in JP-A-2000-303106 and JP-A-2000-355726, when the density of the material before the re-compression forming is less than 7.3 Mg/m³, depending on the re-compression forming method, there is a problem in that a high density and high dimensional precision part is difficult to be obtained.

[0012] It is an object of the present invention to provide an improved producing method of a high density iron based forged part, by which drawbacks encountered in conventional techniques can be effectively overcome.

[0013] Another object of the present invention is to provide an improved producing method of a high density iron based forged part, which makes it possible to produce a high density and high precision iron based forged part at a lower forging load.

[0014] The present inventors, in order to overcome the above problems, have intensively studied sintering conditions and forming conditions, intending to obtain a high density iron based forged part. As a result, it has been found effective to preliminarily forming or compacting a powder mixture followed by sintering at a temperature that allows added graphite to diffuse into a matrix and in a low nitrogen atmosphere, or furthermore followed by applying cold closed die forging or cold enclosed die forging after applying annealing. By this, it is found that even when the density after the preliminary forming is low, a forged part that has a high density and is remarkably improved in the dimensional precision can be obtained. Furthermore, by this, it is found that the forming (forging) after the sintering can be performed under a low forming (forging) load.

[0015] Thus, a method of producing a high density iron based forged part, according to the present invention comprises the following steps in the sequence set forth: (a) preparing iron based powder mixture containing iron based metal powder and graphite powder; (b) preliminarily compacting the iron based powder mixture to form a preliminary compact; (c) sintering the preliminary compact in a non-oxidizing atmosphere whose nitrogen partial pressure is 30 kPa or less, at a temperature of 950° C. or more and of 1300° C. or less to form a forming material; and (d) forging the forming material by closed die forging or enclosed die forging to produce a high density forged part.

[0016] The other objects and features of this invention will become understood from the following description with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

[0017]FIG. 1 is a block diagram showing a typical example of a method of producing a high density iron based forged part, according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0018] According to the present invention, a method of producing or manufacturing a high density iron based (ferrous) forged part comprises the following steps in the sequence set forth: (a) preparing iron based powder mixture containing iron based metal powder and graphite powder; (b) preliminarily compacting the iron based powder mixture to form a preliminary compact; (c) sintering the preliminary compact in a non-oxidizing atmosphere whose nitrogen partial pressure is 30 kPa or less, at a temperature of 950° C. or more and of 1300° C. or less to form a forming material; and (d) forging the forming material by closed die forging or enclosed die forging to produce a high density forged part.

[0019] The production process of the high density iron based forged part, according to the present invention will be discussed in detail hereinafter with reference to FIG. 1.

[0020] As raw material powder for the high density iron based formed part, iron based metal powder and graphite powder, and optionally powder for alloy, are used. The iron based metal powder to be used can be appropriately selected according to the usage, and, though not restricted to a particular one. From a view point of compressibility, in the present invention, an iron based metal powder that has a composition that contains, by mass %, carbon of 0.05% or less, oxygen of 0.3% or less, nitrogen of 0.010% or less and a balance of iron and inevitable impurities can be preferably used as the iron based metal powder. In addition, an oxygen content in the iron based metal powder is preferable to be as low as possible from a view point of compactibility. However, since oxygen is an inevitably included impurity, 0.02% by mass that can be inexpensively and industrially put into practice is preferably set as a lower limit. From the viewpoint of industrial economy, a preferable oxygen content is 0.03 to 0.2% by mass. Furthermore, a nitrogen content in the iron based metal powder is preferable to be as low as possible from a view point of decreasing the forging load. However, from a viewpoint of industrial economy, the nitrogen content is preferably set at 0.010% by mass or less.

[0021] Furthermore, a particle diameter of the iron based metal powder that is used in the invention, though not restricted to particular one, is preferable to be, in terms of an average particle diameter, in the range of 30 to 120 μm that can be industrially manufactured at low cost. The average particle diameter is a value of a mid-point (d₅₀) of a so-called weight cumulative particle size distribution.

[0022] Furthermore, in the invention, in addition to the above composition, as needs arise, one kind or two or more selected from Mn, Mo, Cr, Ni, Cu and V can be contained, and furthermore one kind or two or more selected from Mn: 1.2% by mass or less, Mo: 2.3% by mass or less, Cr: 3.0% by mass or less, Ni: 5.0% by mass or less, Cu: 2.0% by mass or less and V: 1.4% by mass or less can be preferably contained in the iron based metal powder. More preferable contents of Mn, Mo, Cr, Ni, Cu and V are 1.0% by mass or less for Mn, 2.0% by mass or less for Mo, 3.0% by mass or less for Cr, 5.0% by mass or less for Ni, 2.0% by mass or less for Cu and 1.0% by mass or less for V. All of Mn, Mo, Cr, Ni, Cu and V can increase the mechanical strength or the hardenability of the sintered body, so that, as needs arise, these can be selected and contained. These alloying elements may be previously alloyed with the iron based metal powder, or may be partially diffused and adhered to (or partially allowed with) the iron based metal powder thereby forming a partial alloy, or may be mixed with metal powder (alloying powder) for alloy. Among these, the partially alloyed one, being most excellent in the compactibility when compared under the same alloy amount, is preferable. However, in all cases, when Mn, Mo, Cr, Ni, Cu, and V exceed 1.2% by mass, 2.3% by mass, 3.0% by mass, 5.0% by mass, 2.0% by mass and 1.4% by mass, respectively, the hardness of forming material (or material to be formed) becomes higher, resulting in an increase of the forming load at the forging.

[0023] The graphite powder that is used as the raw material powder, with an intention to secure a certain mechanical strength of a forged part or to increase the hardenability at the heat treatment, is preferably contained in an iron based powder mixture (including the iron based metal powder and the graphite powder) by 0.03 to 0.5% by mass with respect to a total amount of the iron based metal powder and the graphite powder. When the content of the graphite powder is less than 0.03% by mass, a strength improvement effect of a sintered body is insufficient, on the other hand, when the content of the graphite powder exceeds 0.5% by mass, a compression load at the forging becomes excessive. Accordingly, the content of the graphite powder in the iron based powder mixture is preferable to be in the range of 0.03 to 0.5% by mass with respect to the total amount of the iron based metal powder and the graphite powder.

[0024] Furthermore, in order to improve a degree of adherence of the graphite powder to a surface of the iron based metal powder, wax, spindle oil or the like may be added to the iron based powder mixture. Furthermore, by applying a segregation preventive treatment disclosed, for instance, in JP-A-1-165701, JP-A-5-148505, a degree of adherence of the graphite powder to a surface of the iron based metal powder can be improved.

[0025] Still furthermore, to the iron based powder mixture, in addition to the above raw material powders, with an intention to improve a compact density at the compacting and to reduce an ejection force for the compact from a die, a lubricant such as a metal soap such as zinc stearate, lithium stearate, and calcium stearate, a higher fatty acid amide such as stearic acid amide, oleic acid amide, and ethylene bis-stearamide, a higher fatty acid such as stearic acid and oleic acid, spindle oil, turbine oil and wax can be added to be contained. A content of the lubricant is preferable to be in the range of 0.1 to 0.6 parts by weight with respect to 100 parts by weight of the total of the iron based metal powder and the graphite powder.

[0026] When the iron based powder mixture is mixed, usually, a known mixing method such as that using a Henshel mixer or a cone mixer can be applied.

[0027] The iron based powder mixture that is mixed at the above ratio is preferably followed by subjecting to preliminary (compression) forming or compacting. In the preliminary compacting, normally well known compacting technology such as a die lubrication method, a multi-stage forming method with a divided die, a CNC press method, a hydrostatic press method, a press forming method disclosed in JP-A-11-117002, a hot forming method, or combinations thereof can be applicable. According to the press forming method disclosed in, for instance, JP-A-11-117002, without heating raw material powder and a die, a compacted body (or compact) having a higher density can be easily manufactured.

[0028] A density of a preliminarily formed or compacted body is preferably set at less than 7.3 Mg/m³. When the density of the preliminarily compacted body is set at less than 7.3 Mg/m³, there is an effect in that restrictions on the conditions of the raw material powder such as the iron based powder being used and so on and on the conditions of the preliminary forming or compacting can be largely alleviated. According to the invention, even when the density of the preliminarily compacted body is less than 7.3 Mg/m³, a forged part having a higher density can be obtained. According to the invention, without depending on the density of the preliminarily compacted body, owing to sintering and forging processes, a forged part having a higher density can be obtained. Furthermore, according to the invention, the lower the density of the preliminarily compacted body is, the larger a density increase of the preliminarily compacted body can be expected. It will be understood that the density of the preliminarily compacted body may be 7.3 Mg/m³ or more.

[0029] Subsequently, the preliminarily compacted body is sintered and supplied as the forming material.

[0030] The sintering is performed in a non-oxidizing atmosphere whose nitrogen partial pressure is 30 kPa or less at a temperature of 950° C. or more and of 1300° C. or less. When the sintering temperature is less than 950° C., the diffusion of graphite into the matrix becomes insufficient. Accordingly, residual graphite, in a recrystallization process, diffuses into the matrix, and thereby disappears and leaves pores, resulting in likelihood of causing strength lowering. On the other hand, even when the sintering temperature exceeds 1300° C., an improvement effect of formability saturates, and by contrast, the manufacturing cost remarkably increases, resulting in being economically disadvantageous. As a result, the sintering temperature is restricted to 950° C. or more and 1300° C. or less.

[0031] In the invention, the sintering is carried out in a vacuum, in an Ar gas, or in an atmosphere that is a non-oxidizing one such as a hydrogen gas and whose nitrogen partial pressure is 30 kPa or less. The lower the nitrogen partial pressure is, the more reduced the nitrogen content in the forming material is, resulting in being advantageous in lowering the forming load at a subsequent cold forging. As a preferable atmosphere, there is a hydrogen-nitrogen gas mixture whose hydrogen concentration is, for instance, 70% by volume or more. On the other hand, when the nitrogen partial pressure exceeds 30 kPa, the nitrogen content in the forming material exceeds 0.010% by mass, resulting in incapability of expecting the above effects. The sintering time, though appropriately determined depending on an object and conditions, is normally preferably set in the range of 600 to 7200 S.

[0032] Furthermore, in the invention, after the sintering, the preliminarily compacted body may be subjected to annealing at a temperature preferably lower than the sintering temperature to prepare a forming material. As a result, the forming material can be appreciably improved in the compression properties (cold forgeability). Although the reasons for this are not necessarily clarified up to the present time, the inventors consider as follows.

[0033] According to the study by the inventors, it is observed that when, after the preliminarily compacted body is sintered to form a sintered body, the annealing is applied, the nitrogen content in the sintered body to be the forming material is lowered. It is read into that since a transformation to an alpha phase in the sintered body proceeds during the annealing and the solubility of nitrogen into the iron alloy matrix becomes lower, a decrease in the nitrogen content in the sintered body is caused. The denitrification due to the annealing is considered to be a factor for improving the compressibility of the forming material.

[0034] Furthermore, the annealing after the sintering is preferably carried out at a temperature in the range of 400 to 800° C. When the annealing temperature is less than 400° C. or more than 800° C., the nitrogen reduction effect becomes smaller. Still furthermore, an atmosphere during the annealing-, similarly to the atmosphere during the sintering, is preferable to be a non-oxidizing one. Furthermore, in order to improve the denitrification efficiency, the nitrogen partial pressure in the annealing atmosphere is preferable to be 95 kPa or less. The nitrogen partial pressure in the atmosphere during the annealing and that in the atmosphere during the sintering are not necessarily required to be the same.

[0035] Still furthermore, the sintering period of time is preferably set in the range of 600 to 7200 s. When the annealing period of time is less than 600 s, the nitrogen reduction effect is slight, and when the annealing period of time exceeds 7200 s, in addition to saturation of the effect, the productivity becomes lower. The more preferable period of time is 1200 to 3600 s.

[0036] Furthermore, there is no problem when the sintering and the subsequent annealing, without taking out the material from a sintering furnace in which the sintering is carried out, are continuously performed. There is neither problem in that after the sintering followed by cooling to 400 to 800° C., the annealing is subsequently applied as it is. Still furthermore, there is neither problem in that after the sintering followed by cooling to less than 400° C., the annealing is applied at a temperature in the range of 400 to 800° C. Furthermore, in the annealing, the temperature is not necessarily held evenly at a definite temperature and may be gradually lowered, for instance, from 800 to 400° C. When gradually cooling, the cooling speed may be lowered so as to take 600 to 7200 s, preferably 3600 to 7200 s, more excessively than a time (about 2400 s) that is necessary to pass the above temperature region at a normal cooling speed.

[0037] Subsequently, the forming material is cold forged, and thereby a forged part is prepared.

[0038] In the invention, the forging is a closed die forging or an enclosed die forging. The “closed die forging” in the invention means the forging in which an almost all surface of the forming material is restrained by a surface of a die so that the material may not be forced out through a clearance of the die, the forging is carried out. Furthermore, the “enclosed die forging” in the invention means the forging in which after the material is confined into the die, the material is pressed by means of a punch or the like, and thereby the material is allowed to fill a space in the die.

[0039] When the forming material obtained according to the above method is subjected to the cold closed die forging or the cold enclosed die forging, a forged part that has a high density and an excellent dimensional precision can be formed at a relatively low forging load. In the closed die forging or the enclosed die forging in the invention, in order to improve the formability or to obtain a further higher density, die lubrication can be preferably applied. The die lubrication can be preferably applied according to an ordinary method in which either a lubricant is coated before the forging or a solid lubricant is used at the forging.

[0040] Furthermore, in the closed die forging or the enclosed die forging in the invention, the die is one that has a closed structure or an enclosed structure, and one in which a certain amount of clearance can be set with respect to the forming material is preferably used. When the clearance is set, since a certain amount of plastic flow can be induced in the forming material at the forging, the density can be further improved.

[0041] The obtained forged part is subjected to a final processing as it is to form a product, or, as needs arise, to re-sintering and/or heating process to form a product.

[0042] The heating process, according to the object, can be selected from a carburizing process, a hardening process, a tempering process or the like. For instance, in the gas carburizing hardening, in an atmosphere whose carbon potential is about 0.6 to 1%, the forged part, after being heated at a temperature in the range of about 800 to 900° C., is preferably subjected to oil hardening. Furthermore, in the bright hardening, in order to inhibit a surface of the sintered body from being oxidized at high temperatures and from being decarbonized, it is preferable that in a protective atmosphere such as an inert atmosphere such as an Ar gas or a nitrogen atmosphere containing hydrogen, the forged part is heated at a temperature in the range of about 800 to 950° C. followed by the oil hardening. Still furthermore, also in vacuum carburizing hardening and in high frequency hardening, the forged part, after being heated at a temperature in the above range, can be preferably hardened. These heat treatments can improve the mechanical strength of a product. Still furthermore, after the hardening is applied, as needs arise, the tempering process may be applied. A tempering temperature is preferably set at a temperature in a normally known tempering temperature range of 130 to 250° C. Before or after the heat treatment, in order to adjust a dimension and shape, machining may be applied to the forged part.

EXAMPLES

[0043] The present invention will be more readily understood with reference to the following Examples in comparison with Comparative Examples; however, these Examples are intended to illustrate the invention and are not to be construed to limit the scope of the invention.

[0044] A certain amount of MoO₃ powder was compounded with atomized pure iron powder (“KIP301A” produced by Kawasaki Steel Corporation) followed by blending by using a V-type blender for 15 min, so that a powder mixture is formed. When the powder mixture was processed in a hydrogen gas stream at 900° C. for 1 h, the MoO₃ powder was reduced and Mo was allowed diffusing to and sticking on a surface of an iron particle, and accordingly partially alloyed iron based metal powder A was formed. According to chemical analysis, it was found that an amount of Mo was 1.0% by mass, in which 1.0% by mass of Mo was partially alloyed. The iron based metal powder A contained 0.15% by mass of Mn as an alloy component which had been previously alloyed.

[0045] Furthermore, according to a water atomization process, iron based metal powder B in which 1.0% by mass of Mo and 0.13% by mass of Mn had been previously alloyed was produced, in which 1.0% by mass of Mo was previously alloyed.

[0046] Both iron based metal powders A and B contained 0.01% by mass of C, 0.15% by mass or less of O, and 0.01% by mass or less of N. Average particle diameters (d₅₀) of the iron based metal powders A and B were in the range of 70 to 80 μm.

[0047] Each of the two kinds of iron based metal powders A and B was blended with graphite powder and a lubricant by using a V-type blender, so that an iron based powder mixture was prepared. As the lubricant, zinc stearate was used. The kinds of the iron based metal powders and contents of the graphite are shown in Table 1.

[0048] The iron based power mixture was filled in a die and, with a forming pressure adjusted by means of a hydraulic compacting machine, was preliminarily formed or compacted, so that a tablet-like preliminarily compacted body having 30 mm diameter and 13 mm height was formed. The density of the preliminarily compacted body is in the range of 6.88 to 7.12 Mg/m³ as shown in Table 1.

[0049] The obtained preliminarily compacted body was sintered under the sintering conditions shown in Table 1, so that a forming material was prepared. The sintering conditions in Table 1 includes kinds of atmospheres in which the sintering was made, nitrogen partial pressure in the atmospheres, temperatures at which sintering was made, and times for which the sintering was made. For some samples (sample Nos. 5 through 9 and Nos. 11 through 16), the sintering was continuously followed by the annealing under annealing conditions shown in Table 1. The annealing conditions in Table 1 includes kinds of atmospheres in which the annealing was made, nitrogen partial pressure in the atmospheres, temperatures at which annealing was made, and times for which the annealing was made.

[0050] Subsequently, the obtained forming material was put in a die having a closed structure and subjected to the cold closed die forging, so that a disc-like forged part having a dimension of 30 mm diameter and 13 mm thickness was produced as a product. The die had a clearance (=inner diameter of the die−outer diameter of the forming material) of 0.4 mm as shown in Table 2. Furthermore, the forging load at the closed die forging was measured. The forgings at the forgings loads of 748 MPa and 1177 MPa were conducted for each sample (each forming material) as shown in Table 2. The density and the hardness of the obtained forged part were measured according to Archimedes method and by use of a Rockwell hardness gauge (B-scale), respectively, as shown in Table 2.

[0051] Furthermore, after the forging was applied, the forged part (product) was observed, and thereby a ratio of an area through which an outer peripheral surface of the product comes into contact with the die to an area of the outer peripheral surface of the die was obtained, so that the transfer properties are evaluated as shown in Table 2. When a value of the ratio is 95% or more, the transfer properties are evaluated as A; 90% or more and less than 95%, B; 80% or more and less than 90%, C; and less than 80%, D. It can be said that the larger the value is, the more excellent the dimensional precision is. The presence of the contact between the outer peripheral surface of the product and the die is judged according to the presence of luster of the outer periphery surface of the product. When the product comes into contact with the die, the luster can be observed on the outer peripheral surface of the product.

[0052] Obtained results of evaluation are together shown in Table 2. As shown in Table 2, Sample Nos. 1, 2, 5, 6 and 10 to 16 correspond to Examples within the scope of the present invention, while Sample Nos. 3, 4 and 7 to 9 correspond to Comparative examples out of the scope of the present invention. TABLE 1 Preliminarily Iron based powder mixture compacted Sintering conditions Iron based Graphite powder body Atmosphere Sample metal powder Content Density Nitrogen partial Temperature Time No. Kind** Kind (% by mass) (Mg/m³) Kind pressure (kPa) (° C.) (S) 1 A Natural 0.3 7.10 Ammonia decomposition gas 25 1100 1800 2 A graphite 0.3 6.92 Ammonia decomposition gas 25 1100 1800 3 A 0.3 7.11 Nitrogen gas 101 1100 1800 4 A 0.3 6.91 Nitrogen gas 101 1100 1800 5 A 0.3 7.09 Ammonia decomposition gas 25 1100 1800 6 A 0.3 6.88 Ammonia decomposition gas 25 1100 1800 7 A 0.3 7.10 Nitrogen gas 101 1100 1800 8 A 0.3 6.92 Nitrogen gas 101 1100 1800 9 A 0.3 6.91 Ammonia decomposition gas 25 900 1800 10 B 0.3 7.11 Ammonia decomposition gas 25 1100 1800 11 B 0.3 7.09 Ammonia decomposition gas 25 1100 1800 12 A 0.3 7.11 Ammonia decomposition gas 25 1100 1800 13 A 0.3 7.10 Ammonia decomposition gas 25 1100 1800 14 A 0.3 7.11 Ammonia decomposition gas 25 1100 1800 15 A 0.3 7.12 Ammonia decomposition gas 25 1100 1800 16 A 0.3 7.09 Ammonia decomposition gas 25 1100 1800 Annealing conditions Atmosphere Sample Nitrogen partial Temperature Time No. Kind pressure (kPa) (° C.) (S) 1 — — — — 2 — — — — 3 — — 4 — — — — 5 Ammonia decomposition gas  25 700 3600 6 Ammonia decomposition gas  25 700 3600 7 Nitrogen gas 101 700 3600 8 Nitrogen gas 101 700 3600 9 Ammonia decomposition gas  25 700 3600 10 — — — — 11 Ammonia decomposition gas  25 700 3600 12 Ammonia decomposition gas  25 300 3600 13 Ammonia decomposition gas  25 400 3600 14 Ammonia decomposition gas  25 600 3600 15 Ammonia decomposition gas  25 800 3600 16 Ammonia decomposition gas  25 900 3600

[0053] TABLE 2 Cold forging Forged part (Forging load 748 MPa) Forged part (Forging load 1177 MPa) Sample Die clearance Density Hardness Transfer Density Hardness Transfer No. mm (Mg/m³) (HRC) properties (Mg/m³) (HRC) properties Remarks 1 0.4 7.45 66.5 A 7.62 79.1 A Example 2 0.4 7.39 67.0 B 7.57 78.8 A Example 3 0.4 7.39 71.0 B 7.56 81.5 A Comparative example 4 0.4 7.34 70.0 C 7.52 82.0 B Comparative example 5 0.4 7.46 64.4 A 7.63 75.9 A Example 6 0.4 7.40 64.3 A 7.59 75.8 A Example 7 0.4 7.44 72.5 B 7.62 75.5 A Comparative example 8 0.4 7.39 69.5 B 7.58 80.5 A Comparative example 9 0.4 7.30 80.0 D 7.51 90.2 C Comparative example 10 0.4 7.35 69.7 C 7.52 82.9 B Example 11 0.4 7.40 67.5 B 7.59 79.6 A Example 12 0.4 7.42 66.5 A 7.62 78.5 A Example 13 0.4 7.45 65.3 A 7.63 77.2 A Example 14 0.4 7.46 64.5 A 7.62 76.0 A Example 15 0.4 7.47 63.5 A 7.65 69.5 A Example 16 0.4 7.43 66.0 A 7.61 78.0 A Example

[0054] Examples (Sample Nos. 1, 2, 5 and 6) are higher in the density (that is, can be forged under a lower load) and excellent in the transfer properties (excellent in the dimensional precision) as compared with Comparative examples (Sample Nos. 3, 4, 7 and 8) in which the sintering was made under-a higher nitrogen partial pressure, when formed under the same forging load. Furthermore, Examples (Sample Nos. 5 and 6) in which the annealing was applied after the sintering are higher in the density and excellent in the transfer properties as compared with Examples (Sample Nos. 1 and 2) in which no annealing was made, when formed under the same forging load. Furthermore, Examples (Sample Nos. 13, 14 and 15) in which the annealing was made at a temperature in the range of 400 to 800° C. are higher in the density and excellent in the transfer properties, as compared with Example (Sample No. 12) in which the annealing was made at 300° C. and Examples (Sample Nos. 15 and 16) in which the annealing was made at 900° C., when formed under the same forging load.

[0055] Furthermore, Examples (Sample Nos. 1 and 5) in which the partially alloyed iron based metal powder A is used are higher in the density and excellent in the transfer properties, as compared with Examples (Sample Nos. 10 and 11) in which the preliminarily alloyed iron based metal powder B was used, when formed under the same forging load.

[0056] As appreciated from the above, according to the present invention, a higher density iron based forged part can be produced or manufactured at a low forging load and additionally with higher dimensional precision. Accordingly, significant industrial benefits can be attained by the production method fo the present invention.

[0057] The entire contents of Japanese Patent Application No. P2002-054244, filed Feb. 28, 2002, is incorporated herein by reference. 

What is claimed is:
 1. A method of producing a high density iron based forged part, comprising the following steps in the sequence set forth: preparing iron based powder mixture containing iron based metal powder and graphite powder; preliminarily compacting the iron based powder mixture to form a preliminary compact; sintering the preliminary compact in a non-oxidizing atmosphere whose nitrogen partial pressure is 30 kPa or less, at a temperature of 950° C. or more and of 1300° C. or less to form a forming material; and forging the forming material by closed die forging or enclosed die forging to produce a high density forged part.
 2. A method as claimed in claim 1, wherein the step of preparing the iron based powder mixture includes partially diffusing at least one metal selected from the group consisting of Mn, Mo, Cr, Ni, Cu and V to be adhered to the iron based metal powder.
 3. A method as claimed in claim 1, further comprising annealing the forming material after the step of sintering the preliminary compact.
 4. A method as claimed in claim 3, wherein the annealing is carried out at a temperature within a range of between 400° C. and 800° C.
 5. A method as claimed in claim 1, wherein the preliminary compact has a density of 7.3 Mg/m³ or less. 